The Kikai Caldera is a mostly-submerged volcanic complex off the coast of Kyushu in Japan's Kagoshima prefecture, and scientists have reportedly discovered what they thought was the biggest eruption ever discovered to take place in the Holocene, the current epoch Earth is in.

According to a Journal of Volcanology and Geothermal Research study by a team of researchers from Kobe University, the volcano has erupted more than once throughout its active history, but accurately assessing the size of each blast has been tricky.

To determine the size of an event that occurred around 7,300 years ago, known as the Kikai-Akahoya (K-Ah) eruption, the researchers combined sediment samples from the seabed with seismic imaging to map out the shape of the caldera and the materials in it, looking for evidence of volcanic ejecta.

"Large volcanic eruptions such as those yet to be experienced by modern civilization rely on sedimentary records, but it has been difficult to estimate eruptive volumes with high precision because many of the volcanic ejecta deposited on land have been lost due to erosion," Kobe University geophysicist and research team member Nobukazu Seama said.

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The study revealed that the team focused on what was called the pyroclastic flow of the K-Ah eruption, specifically the volume of ash, rock, and gas that would have powered through the water several millennia ago.

While the flow is hard to model underwater, water does a good job of preserving the volcanic ejecta. The researchers used instruments to look deep below the seafloor, comparing different types of material in terms of where they originated and where they ended up.

Science Alert reported that the team found the material ejected by the eruption would've covered around 4,500 square kilometers, an area several times bigger than large metropolitan cities like London or Los Angeles.

The K-Ah eruption allegedly sent hundreds of cubic kilometers of rock and dust scattering, making it the largest Holocene eruption ever measured, even dwarfing the 1883 eruption of Krakatoa.

"By using seismic reflection surveys optimized for this target and by identifying the collected sediments, we were able to obtain important information on the distribution, volume, and transport mechanisms of the ejecta," Kobe University marine geophysicist Satoshi Shimizu said.

The research aimed to give experts a better idea of how volcanic eruptions will pass through water in the future and help in volcano modeling.

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